Making a semiconductor laser output a super-high intensity pulse has come to the fore as an important issue even in a sense of searching new physical phenomenon as well as only the pursuit of extreme performance of the laser (see Non Patent Literatures 1 and 2 below).
As a method of generating a short pulse in the semiconductor laser, as illustrated in FIG. 1, there is a known gain-switching operation using a relaxation oscillation mechanism (see Non Patent Literature 3 below).
As an application of such a technology, there is a technology of shortening the pulse width of an optical pulse. In this technology, when a laser diode (hereinafter, referred to as LD) generates an optical pulse in a bandwidth broadened by chirping, a signal in a bandwidth narrower than the width broadened by the chirping is extracted from optical spectra of the optical pulse generated by the laser diode (see Patent Literature 1 below).
In such a technology, a band pass filter is used only for the purpose of extracting a component of the optical pulse in the narrower bandwidth from the optical spectra broadened by the chirping.
Further, there is a technology in which, after being compressed by a pulse compressor in compensation with down chirping, an optical pulse from a single longitudinal mode semiconductor laser is amplified by an amplifier, subjected to a process of removing noises due to spontaneous emission light included in the pulse compressed by a waveform shaper and a pedestal component, and further amplified by another amplifier to gain short pulse light (see Patent Literature 2 below).